(1) Field of the Invention
The present invention relates to optical laser systems and, in particular, to mode-locked laser systems having free-space feedback couplers.
(2) Description of the Prior Art
High average and high peak power lasers such as diode or fiber lasers with good beam quality are attractive sources for applications including high energy lasers for use in directed energy systems, industrial laser machining, and numerous scientific applications. For scaling the output power of the fiber based laser systems (e.g., to hundreds of kilowatts), as is required for directed energy applications, some method to provide optical amplification is required.
In fiber amplifiers, the gain medium is a glass fiber that can be doped with rare earth ions such as erbium, neodymium, ytterbium, praseodymium, or thulium. The active dopant is pumped by an optical source, such as a fiber-coupled diode laser, where the pump light propagates through the fiber core along with the signal being amplified.
Raman amplifiers, unlike that described in the present invention, are based on Raman gain resulting from stimulated Raman scattering (SRS). In fiber Raman amplifiers, where the nonlinear active medium is typically an optical fiber, the maximum gain is obtained for a frequency offset between pump and signal depending on the composition of the fiber core. Fibers used for Raman amplifiers are not doped with rare earth ions. Any typical single mode fiber can practically be used. Compared to rare earth (e.g., erbium) doped fiber amplifiers, Raman amplifiers have the following advantages and disadvantages. The SRS amplification process can be readily cascaded, thus accessing a variety of wavelength regions if suitable pump sources are available. In addition, the gain spectrum can be tailored by using different pump wavelengths simultaneously. Raman fiber amplifiers require a longer length of fiber but can have a lower noise figures. Lastly, if the pump wavelength is polarized, the Raman gain will be polarization dependent, which is often an unwanted effect.
Beam combination can be utilized for scaling laser output power. This, however, generally requires single frequency operation for each of the stages. Wavelength beam combining (WBC), however, can be an incoherent process where the brightness scales proportionally to the total number of laser input elements. The output beam of a WBC system is that of a single multi-wavelength beam, where the output power scales the power from each of the input laser elements.
WBC allows for brightness scaling of a diode laser array because all of the laser elements are spatially overlapped at the output coupler, maintaining the output beam quality (low M2 value) of a single element while scaling the output power by the number of elements in the array. WBC can be applied to any laser with a gain bandwidth, including fiber lasers.
A primary limitation for scaling single-frequency fiber lasers and amplifiers to higher power and higher energy is due to stimulated Brillouin scattering (SBS). SBS results from nonlinear optical effects in the fiber. It primarily occurs when narrowband optical signals are amplified in a fiber amplifier. This leads to vibrations in the optical medium that cause scattering. Thus, there is a need for an improved diode or fiber laser that can be used at high power and high energy levels.